Driver circuits for the operation of LEDs are known in principle from the prior art. Such a driver circuit is supplied from an electrical power source and comprises a resonant circuit, for example, an LLC converter which is responsible for transmitting current via a galvanic separation or respectively galvanic barrier from a primary side to a secondary side of the galvanic barrier. The purpose of this transmission of electrical energy is to supply an LED series connected to the secondary side with current.
It is known that such a resonant circuit or respectively such a driver circuit is operated as a constant-current converter. For this purpose, a control loop can be provided for the control of the LED current, wherein an actual value of the LED current can be measured on the secondary side of the galvanic barrier. However, this actual-value measurement must be fed back to a primary-side control loop or respectively a primary side control circuit, in order to control the driver circuit correspondingly. The disadvantage with this embodiment is the fact that the feeding back of the actual value of the LED current to the primary-side control circuit requires a potential separation and consequently an optocoupler.
Accordingly, attempts have already been made in the prior art to dispense with a secondary-side detection of the LED current and to detect the value of the current indirectly via the current on the primary side of the transformer. Starting from this feedback value, a current control is implemented, in which a corresponding switch of a half-bridge circuit of the driver circuit is varied.
However, even such embodiments are not satisfactory. In particular, this detection on the primary side has the disadvantage that it does not directly reproduce the LED current, but, on the contrary, also comprises the variable component of the magnetisation current on the primary side. This component is variable and depends, in particular, upon the LED voltage, which, in turn, is dependent, for example, upon the number of LEDs. A fixed compensation of the magnetisation current cannot therefore take place. On the contrary, a direct measurement of the LED voltage is necessary for the compensation, which, however, in turn, requires an AD converter and a feedback via an optocoupler to the control circuit on the primary side.